Along with widespread adoption of smartphones in recent years, insufficiency in base station capacity has become one of the problems. As a countermeasure for solving insufficient base station capacity, a small-cell base station architecture using a number of small-size base station apparatuses is being discussed. Such architecture is expected to become popular in the coming years.
To enhance the base station capacity, active antenna systems (AASs) using vertically arranged antenna elements are also discussed. Elevation beamforming allows vertical sectorization (dividing a cell into a closer sector and a more distant sector from the base station) to improve the capacity.
Three-dimensional multiple input multiple output (3D-MIMO) systems using horizontally and vertically arranged antenna elements are being put into practical use. In MIMO systems, correlation between antenna branches depends on the radio environment to some extent. In other words, the correlation may vary depending on the environment even though the antenna configuration is the same. For example, in urban areas with more reflected waves, the angle spread of radio waves (or distribution of angle of arrival) is greater and the correlation between antenna branches becomes smaller. In order to produce high-quality areas, it is desired to determine the optimum antenna configuration reflecting the radio environment and set up an antenna that meets with the optimum configuration.